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Late blight outbreaks are springing up in potato crops after the wet summer continued into August following a dismal July. Wet summer sees increase in potato blight outbreaks

• Big rise in incidence this summer

• Growers urged to remain vigilant

• Still a risk, so don’t be complacent

Late blight outbreaks are springing up in potato crops after the wet summer continued into August following a dismal July.

The UK had 170% of its usual July rainfall, making it the wettest July since 2009 and the sixth wettest on record, said the Met Office. Predictably, this has led to a significant rise in blight outbreaks nationwide.

‘No respite’

The Fight Against Blight monitoring service – run by the James Hutton Institute – reported 120 crop outbreaks to the middle of August. This puts 2023 on track to be a high-pressure year alongside 2019, 2012 and 2007.

Outbreaks are spread around all regions, with no respite for drier areas such as East Anglia. James Hutton research leader David Cooke said: “It has been a challenge to keep on top of the samples coming into the lab.”

Dr Cooke isolates and characterises each sample as it arrives, also undertaking a DNA analysis. The aim is to spot any changes in the blight population as the season progresses so spray programmes can be adapted based on the present genotypes.

Big concern

The big concern is the EU-43-A1 genotype, which is resistant to the carboxylic acid amide (CAA) group of fungicides, including mandipropamid. So far this season, it has not been found so far this season – but there is no cause for complacency.

That does not mean blight risk is low because on of the dominant strains is 36-A2.

Present for a number of years, it is extremely aggressive and can pose a serious challenge if it takes hold in a crop.

“Trying to firefight blight is very difficult – but the chemistry must continue to be used wisely, especially concerning the longer-term risk of resistant genotypes,” explains Dr Cooke.

“We do not want other actives struggling under high pressure being misused. It is tempting for growers to keep using the products that will give them the best control, but they must use a range of fungicides.

Good news

“The good news is that the canopy has finished growing, so at least when a fungicide is applied, it protects all leaves for a time. I would encourage growers to keep an eye on the lower canopy to ensure there is no residual amount of blight there.”

This could be the case following frequent heavy rainfall, which will spread the disease down the canopy. The worry is that the conditions have also suited tuber infection from any foliar blight in the crop.

Lower temperatures will encourage the production of zoospores, and combined with soil moisture, there is a high risk that if foliar blight remains unchecked, it will quickly become an issue in the soil, says Dr Cooke.

By now, most crops are at or beyond the “canopy complete” stage, says UPL potato technical expert Geoff Hailstone.

“When outbreaks are found in the field, the priority must be to clear these up and stop them spreading as soon as possible.”

Where this is the case, Mr Hailstone says a tank mix should include Proxanil (cymoxanil + propamocarb).

Controlling outbreaks

“Cymoxanil is known to be one of the few actives with kickback activity and has a very low risk of developing resistance.

Propamocarb has strong anti-sporulant activity, good movement in the plant and is only active in the carbamate resistance group, says Mr Hailstone.

“When tank-mixed with a protectant fungicide such as cyazofamid, Proxanil strengthens the activity and gives excellent resistance management. Where blight appears in the crop, there should ideally be two closely timed sprays, while adhering to label intervals.”

If foliar blight is active in the canopy, especially if conditions are favourable, then the risk of resistance can be high. Mr Hailstone recommends mancozeb for resistance management and to protect uninfected leaves.

Controlling outbreaks

Although mancozeb only has
protectant activity, it still has a role in controlling outbreaks when mixed with curative products. Mancozeb is sold as a straight product in Manzate 75WG or formulated with cymoxanil in Nautile DG (cymoxanil + mancozeb).

“Hopefully, the weather turns more settled until harvest and growers can keep on top of disease in their potato crops – but they must maintain blight programmes until the foliage and stems are dead.

“Where a grower knows that foliar blight has been present in the crop, they should study the susceptibility to tuber blight of the variety they are growing and prioritise harvest accordingly,” concludes Mr Hailstone.